Audio systems with vacuum tube amplifiers are still commercially available even though most modem audio systems typically use solid state transistors. Nonetheless, many people still prefer vacuum tube amplifiers because they enjoy the sound produced by the vacuum tube amplifiers, because they enjoy the lights of the vacuum tubes, or for other reasons. One type of popular vacuum tube amplifier uses a push-pull circuit.
In a push-pull circuit one vacuum tube amplifies the positive half of an input signal while another vacuum tube amplifies the negative half of the input signal. Both halves of the signal are ultimately combined in the secondary of an output transformer. The secondary provides power to the speaker-load typically at high currents and low voltages. A conventional push-pull output transformer comprises three windings wound around a magnetic core: a half primary winding for each half of the input signal and a secondary winding for the speaker load.
The output transformer has limited the usefulness and applicability of the push-pull amplifier because the output transformer limits frequency response at the upper and lower ends of the audio spectrum, and also introduces notch distortion. In order for an output transformer to respond properly at low frequencies, a large number of turns in the primary is needed to produce a large inductance. Unfortunately, a large number of turns in the primary increases the distributed capacitance between the windings and also increases leakage inductance, both of which effect high frequency response. Thus, while increasing the number of turns in the primary improves performance for low frequencies, it sacrifices performance at high frequencies.
Another problem introduced by the output transformer when used in an amplifier operating the output tubes class AB2 or class B is notch distortion. Notch distortion cannot be eliminated by overall feedback. Vacuum tubes in push-pull arrangements such as Class AB, or B are more efficient than class A amplifiers, but notch distortion can occur at the point where one of the tubes stops conducting and the other tube begins conducting. Notch distortion is due to imperfect coupling between the two halves of the primary when the impedance of the source of the primary is high. Notch distortion does not usually show up below 1000 Hz and becomes excessive starting at about 3000 Hz.
In 1949, Macintosh disclosed a "unity coupled circuit" that allowed the output tubes to operate in parallel through a bifilar winding, effectively eliminating notch distortion. But, the unity- coupled circuit requires extensive positive feedback to overcome degenerative cathode swings causing problems yet more difficult to solve. Other attempts to reduce source impedance include the single ended push-pull circuit, the Peterson Sinclair circuit, and the Wiggins Circlotron circuit.
Another way to eliminate notch distortion would be to provide a transformer for a conventional push-pull circuit that is tightly coupled between the two half primaries. It is generally felt that a ratio of the open circuit primary inductance to the leakage inductance of 80,000:1 would substantially eliminate notch distortion. It is therefore desirable to provide a transformer with reduced leakage inductance that can accomplish this 80,000:1 ratio.
Bifilar windings of the two half primaries are known in the art to reduce leakage inductance, but these bifilar windings have introduced problems into output transformer design. One problem is that high AC potential exists between adjacent wires of the bifilar windings, so the wires must be adequately insulated to withstand the potential. Also, bifilar windings create considerable capacitance between adjacent wires, and that capacitance must be charged in developing potential difference between the wires. The charging current must be supplied by the output tubes, and this limits the high frequency power output of the amplifier.
In a transformer with bifilar primary windings, each winding has capacitance with respect to the two windings on each side of it in the same layer, and also with respect to windings in the layers above it and below it. Effective capacitance between windings in the same layer may be cut in half by transposing windings of the bifilar pair at every turn. Capacitance between wires in adjacent layers may be reduced by increasing the spacing between layers, but this increases the leakage inductance of the transformer.
Thus, in order to improve performance at the upper and lower ends of the audio system and to reduce notch distortion, it is desirable to provide a transformer with sufficiently low leakage inductance (i.e., a ratio of open circuit primary to leakage inductance of greater than 80,000:1) without substantially increasing distributed capacitance. In other words, it is desirable to increase coupling between the windings without increasing the capacitance.